This invention relates to the use of borohydrides in hydrogen storage and release technologies. Borohydrides such as LiBH4 can be used for hydrogen storage and energy systems making use of stored hydrogen. Borohydrides contain a large amount of hydrogen within their molecular structure. For example, LiBH4 contains 18 wt % hydrogen, an amount higher than any other known hydrogen storage material. Accordingly, borohydrides have great potential to be developed as hydrogen storage media.
Unfortunately, borohydrides release hydrogen at very high temperatures, with temperatures usually exceeding a melting point of the borohydrides. For example, commercially available LiBH4 releases hydrogen above 400° C. In addition, the hydrogen release mechanism is typically irreversible for commercially available LiBH4 in that the borohydride cannot be rehydrided.
It is known to use various borohydrides for specialized applications requiring a hydrogen storage material. For instance, U.S. Pat. No. 6,737,184 assigned to Hydrogenics Corporation, and which is incorporated herein by reference, discloses one release mechanism using LiBH4 in which a solvent such as water is used to bring about the release of stored hydrogen. However, once released, the LiBH4 cannot be easily rehydrided.
Similar aqueous based release reactions for borohydrides may also be seen in reference to U.S. Pat. Nos. 6,670,444; 6,683,025; and 6,706,909 all assigned to Millennium Cell and which are incorporated herein by reference. The cited references are all directed to aqueous-based reactions for releasing hydrogen from a borohydride. There is no discussion within the references of catalysts or material handling techniques that allow the reversible release of hydrogen from a metal borohydride containing solid compound.
It is also known in the art that borohydrides may release hydrogen through a thermal decomposition process. For instance, in U.S. Pat. No. 4,193,978 assigned to Comphenie Francaise de Raffinage and which is incorporated herein by reference, lithium borohydride is described as a hydrogen storage material which releases hydrogen during a thermal decomposition process. The reference stated that aluminum may be added to the lithium borohydride to lower the reconstitution temperature and to increase the hydrogen capacity of the material. There is no discussion of catalysts or other materials or techniques designed to bring about a lower hydrogen release point temperature.
It has been reported in the article, “Hydrogen Storage Properties of LiBH4”, Journal of Alloys & Compounds, 356-357 (2003) 515-520 by Zuttlel et al and which is incorporated herein by reference, that LiBH4 may include a low temperature structure of an orthorhombic, space group having a hydrogen desorption value reportedly occurring at approximately 200° C. in the presence of SiO2. However, an ability to rehydride the lithium borohydride and the use of additives other than the SiO2 in reducing the dehydriding temperature and isothermal dehydriding properties are not reported.
Currently, the art recognizes that borohydrides, when subjected to high temperatures, may decompose and release hydrogen at a point near or in excess of the borohydride's melting point. Alternatively, borohydrides can also be used through a hydrolysis process to provide a source of hydrogen. However, there remains room for improvement and variation within the art directed to the use of borohydrides in hydrogen storage applications.